What wire insulation is indicated by the letter 'R'?

The letter 'R' in wire insulation stands for rubber. Rubber insulation is valued for its flexibility and excellent electrical properties, making it suitable for a wide range of applications in electrical wiring. It provides good resistance to moisture, abrasion, and certain chemicals, which makes rubber-insulated wires ideal for outdoor and industrial use where the conditions may be harsh.

In contrast, other insulation materials listed have different properties and applications. Asbestos, while historically used for its heat resistance, is now largely phased out due to health concerns. Thermoplastic insulation has excellent heat resistance and can be molded, but it may not offer the same level of flexibility and durability under temperature variations as rubber does. Cross-linked synthetic polymers, while providing strong performance, have different characteristics and are usually designated with different letters. Rubber continues to be a standard choice in many electrical applications due to its longstanding reliability and safety.